That "not secure" message can be completely accurate, but it can also be misleading and unhelpful. For example, it will appear if your iPad cannot maintain a secure connection with the website server due to poor wireless and related network conditions. All Safari knows is that it was unable to negotiate a secure connection. It doesn't know why.
First, restart your iPad in the usual manner: Restart your iPad - Apple Support. Sometimes it fixes this very problem.
If that doesn't help, clear Safari's history and website data: Clear the history, cache, and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support.